At first time, trying to check this relay modules (I got 2 the same) they both worked nice - no problem.


But now I am going tu build the temperature controller for old fridge (it is working constantly). I got the problem: both LEDs (red indicates relay being ON and green - indicates POWER ON) are working, but there is no connection between NC and COM.

Does not matter HIGH or LOW jumper position.

At first tests I wasn't used any resistors, and everything was fine. Voltage between DC+ and DC- are about 4.7 V. Between DC- and IN - about 4.5V.

At first I was sinking about not enough current form Arduino NANO with miniUSB, but the same result.

So, I have this schematic (have tryied some pins - doesn't affect anything.


simulate this circuit – Schematic created using CircuitLab

I have tested using this code:

int testpin = 7;

void setup() {                
  pinMode(testpin, OUTPUT);// initialize the pin as an output

void loop() {
  digitalWrite(testpin, HIGH);//turn on
  delay(5000);// wait for 5000 milliseconds (five seconds)
  digitalWrite(testpin, LOW);//turn off
  delay(5000);//wait five seconds

So, have I burned my relays? If I use 4 1,5 batteries to check it working - it cannot damage anything?

How exactly wire this relay?

Should I try to use solid-state relay (and how to manage it working)?

  • \$\begingroup\$ "... there is no connection between NC and COM." That indicates a problem with either the board or the relay itself. Have you checked the continuity of both directly? \$\endgroup\$ Jul 25, 2015 at 18:17
  • \$\begingroup\$ No connection between NC and COM is exactly what one would expect with the relay actuated! NO and COM should show continuity. \$\endgroup\$ Jul 25, 2015 at 18:31
  • \$\begingroup\$ @Ignacio what do you mean with "check the continuity of both directly"? Connect 5+ to the DC+ and IN? I have tried - only LEDs are ON, but relay is not switching NC and NO. \$\endgroup\$
    – XuMuK
    Jul 25, 2015 at 19:00
  • 1
    \$\begingroup\$ Take your DMM, put it in continuity mode, and test to see if the PCB's traces are unbroken and that there is continuity from COM to NC when the relay is unpowered. \$\endgroup\$ Jul 25, 2015 at 19:03
  • \$\begingroup\$ @Spehro it has working LEDs, but relay is not switching. \$\endgroup\$
    – XuMuK
    Jul 25, 2015 at 19:11

1 Answer 1


Thanks Ignacio Vazquez-Abrams and Spehro Pefhany for your advices!

So, I have reconnected all wires and seems one relay I got is broken now (I suppose I have done short circuit)...

Now, when the signal is ON I have connection between NC and COM.


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